Four Transocean workers have refused to appear before the Coast Guard-Interior Department inquiry into the sinking of the Deepwater Horizon drill rig, forcing the panel to cancel a Wednesday hearing into the worst oil spill in U.S. history.

All four worked on the doomed rig’s blowout preventer, the massive fail-safe device that has failed to cut off the undersea gusher at the heart of the disaster. They “declined to voluntarily appear” before the joint Coast Guard-Interior Department board investigating the sinking, the panel said in a statement canceling the hearing.

Coast Guard Petty Officer Elizabeth Bordelon said the men are from the Houston, Texas, area, as are many of the BP and Transocean employees slated to testify in upcoming hearings, and cited problems with travel arrangements for their cancellation. The rest of the week’s proceedings are expected to go on as planned, and the men’s testimony has been rescheduled for August in Houston, where the board plans its next round of hearings, she said.

“We don’t anticipate any problems for the rest of the week or the future set of hearings,” said Bordelon, a spokeswoman for the investigative board.

Transocean owned the Deepwater Horizon, which sank two days after an April 20 explosion off Louisiana that left 11 crew members dead. It leased the platform to BP, the owner of the ruptured well, and BP, Transocean and cementing contractor Halliburton all have blamed each other for the disaster.

The investigative panel is charged with finding the cause of the disaster “to the fullest extent possible” and could refer any findings of misconduct to federal prosecutors. The panel has a January 2011 deadline for completing its report, Bordelon said.